Loans modifies in a troubled debt restructuring individually evaluated for impairment principal balance threshold.

No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Represents the maximum maturity period for cash equivalents.

+ References

No definition available.

+ Details

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- Definition

Represents the number of investment portfolios.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of securities held as trading securities.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
Data Type:	xbrli:integerItemType
Balance Type:	na
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- Definition

Transition adjustment increase in loan loss reserves.

+ References

No definition available.

+ Details

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- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5111-111524

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8672-108599

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Balance Type:	na
Period Type:	duration

- Definition

Describes the periods of time over which an entity anticipates to receive utility from its property, plant and equipment (that is, the periods of time over which an entity allocates the initial cost of its property, plant and equipment).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 35
-Paragraph 4
-URI https://asc.fasb.org/extlink&oid=126905813&loc=d3e1205-110223

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Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) Portfolio TradingSecurities Segment	Dec. 31, 2021 USD ($) TradingSecurities	Jan. 01, 2023 USD ($)
Finite Lived Intangible Assets [Line Items]
Maximum maturity period for cash equivalents	90 days
Number of investment portfolios \| Portfolio	3
Number of securities held as trading securities \| TradingSecurities	0	0
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	$ 1,700	$ 2,000
Useful lives of premises and equipment	three to forty years
Number of reportable segments \| Segment	2
Core Deposits [Member]
Finite Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	7 years
Subsequent Event [Member]
Finite Lived Intangible Assets [Line Items]
Transition adjustment increase in loan loss reserves			$ 2,400
Minimum [Member]
Finite Lived Intangible Assets [Line Items]
Loans Modified in a Troubled Debt Restructuring, Individually Evaluated for Impairment, Principal Balance, Threshold	$ 500
ALC [Member] \| Minimum [Member]
Finite Lived Intangible Assets [Line Items]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	$ 50